ABSTRACT Upon infecting a pregnant woman, Zika virus (ZIKV) may cross the placenta, infect the fetus and result in abroad array of birth defects including microcephaly, growth restriction, fetal demise, arthrogryposis, and vision and hearing (congenital Zika syndrome (CZS)) as seen with over 2500 cases 2015. The risk factors for CZS are among the many unknowns in the wake of this epidemic. The hypothesis and major concern is that maternal antibodies (Ab) elicited by prior dengue (DENV) infection may cross-react with ZIKV and exacerbate disease phenotypes similar to the Ab-dependent enhancement (ADE) observed in severe, secondary DENV infections. Determining whether DENV immunity is a risk factor for CZS is the imperative next step. Specific aim 1 of this proposal will determine whether DENV immune status is a risk modifier for CZS. In order to efficiently generate reliable epidemiologic data, a case-control study will be conducted by leveraging ongoing cohort studies in Nicaragua and Colombia. The DENV-immune status will be characterized by unique methods available in our lab permitting the detection of type-specific antibodies that reflect an individual?s flavivirus exposure history. As a sub-aim, we will validate cord blood as a convenient sample for performing serologic assays that reflect the mother?s flavivirus antibody responses. Because little is known about fetal the immune response to congenital ZIKV, we will assess the function and phenotype of fetal T and B cells from cord blood in Specific aim 2. It is known that T cell phenotype is altered in congenital cytomegalovirus infection, I predict that a greater frequency of effector and memory T cells will be present in cord blood from infants with CZS compared to uninfected controls. A greater capacity to secrete effector cytokines is also likely to be observed following polyclonal stimulation. Antigen-specific T cell responses will be quantitated by measuring cytokine secretion to ZIKV- derived peptides; antigen-specific B cells will be quantitated by modified ELISPOT techniques following activation of fetal memory B cells. Specific aim 1 promises to yield practical knowledge for improving public health and clinical approaches for monitoring ZIKV. It also provides crucial information to maintain safety in development and implementation of vaccines to DENV and ZIKV. Aim 2 may identify immune correlates of pathology or infection that hold diagnostic or prognostic utility. Targets for immune-based interventions to protect ZIKV-exposed pregnancies may also be discovered.